The present invention relates to an optically anisotropic film used for super twisted nematic (STN) type or electric field-controlled birefringence (ECB) type liquid crystal displays and the like, a process for the production thereof, and a liquid crystal display device using said film.
A retardation film can be obtained by uniaxially stretching a transparent thermoplastic polymer film and is used as an optical compensator (color compensator) to compensate for tinting in especially STN type or ECB type liquid crystal displays to improve their display performance. The liquid crystal display devices using such retardation films have advantages such as light weight, small thickness and low cost.
Recently, the opportunity of use of liquid crystal devices under a high-temperature condition has increased. For instance, they are adapted in a car navigation system which is set in an automobile for providing a visual geographical information and is gaining popularity among motorists.
Request is rising for application of relatively inexpensive STN type liquid crystal display to such a use. However, use of such a liquid crystal display under a high temperature condition involves a problem of reduced display performance because of difference in retardation between the STN liquid crystal cell and the retardation film at high temperatures.
Further, with enlargement of image area of the STN type liquid crystal displays, a problem has arisen in that the temperature in the liquid crystal cell may become non uniform due to heat conducting from the fluorescent tube used for backlight, resulting in different display performance in an image area.
Nevertheless, there is not yet available any optically anisotropic films which can meet both requirements for display quality and viewing angle characteristics when used in an environment which may be exposed to high temperatures, such as the interior of an automobile.
The reduction of display performance of the liquid crystal display devices at high temperatures may be attributed to the fact that generally the alignment of the liquid crystal molecules or polymer is relaxed as the ambient temperature rises. In this case, the degree of alignment relaxation of the low-molecular weight liquid crystal material used for the liquid crystal cell becomes higher than that of the polymer used for a retardation film, resulting in a greater change of retardation, so that the retardations in said two members, which has been set to be optimized at room temperature, deviates from the optimal level under high temperatures, resulting in imperfect color compensation of the liquid crystal cell and thus causing tinting and other problems leading to a reduction of the display performance. Thus, an optically anisotropic film which allows variation of retardation in conformity to the change of birefringence of the liquid crystal cell with temperature has been required.
Various types of optically anisotropic films using a liquid crystalline material have been proposed.
JP-A-3-291601 discloses a process for producing a retardation film, wherein in order to obtain a large-area and uniform retardation film, a solution having dissolved therein a liquid crystalline polymer substantially comprising a polyester having the ortho-substituted aromatic structural units is applied on a substrate having a known alignment film and subjected to a heat treatment to effect horizontal alignment.
The optical retardation in a retardation film is given as the product .DELTA.n.multidot.d of the refractive anisotropy of the film material (.DELTA.n) and the film thickness (d). In a retardation film in which only those of the liquid crystalline polymer molecules having large anisotropy of refractive index have been horizontally aligned, it needs to control the liquid crystalline polymer film thickness very precisely for obtaining uniform retardation throughout the film. This is, however, difficult and impractical in industrial production.
JP-T-4-500284 discloses a retardation film having the same temperature dependence and wavelength dependence as the liquid crystal molecules used in the liquid crystal cell, saod film comprising a side-chain type liquid crystal polymer having a linear or cyclic backbone. It is, however, difficult to make highly precise control of thickness of a film made of a side-chain type liquid crystal polymer with large anisotropy of refractive index.
JP-A-5-257013 discloses a retardation film obtained by dispersing liquid crystal molecules in a polymer film and stretching this film. But this reference is silent on what combination of liquid crystal compound and polymer with provide the desired effect.
The optically anisotropic films disclosed in the above references are indeed effective for reducing viewing angle dependence, but since stretched films of polymers having a high glass transition temperature are used, the change of retardation with temperature is very limited and insufficient for compensating temperature dependence of birefringence of the liquid crystal cell.
In the case of the films using a polymer liquid crystal, since the polymer liquid crystal is first aligned and then its alignment is fixed by cooling, it is necessary to use a polymer liquid crystal whose transition temperature from the liquid crystal phase to the glass phase is well higher than room temperature. However, since the glass transition temperature of crystallization temperature of the liquid crystal used in the liquid crystal cell is below room temperature, the temperature compensating effect is unsatisfactory.
JP-A-8-190094 discloses an optically anisotropic film mainly comprising a mixture of a transparent or semi-transparent polymer and a liquid crystal compound, wherein retardation of the optically anisotropic film at 80.degree. C. is 20% to 97% of retardation at 30.degree. C., and the content of the liquid crystal compound is 0.5% to 50% by weight based on the total weight of the liquid crystal compound and the polymer. It is also described that the liquid crystal compound may be a copolymer of liquid crystal oligomers having different side-chains.
Examples of liquid crystalline compounds which can be used in temperature controlled optically anisotropic films are liquid crystalline polysiloxanes. The properties of such polysiloxanes are described in the literature (e.g. C. Burger and F. H. Kreuzer, "Polysiloxanes and Polymer Containing Siloxane Groups" in "Silicon in Polymer Synthesis", Ed. H. R. Kricheldorf, Springer-Verlag, Berlin, Heidelberg, 1996, and references cited therein). It is known from a work of G. -P. Chang Chien et al. (J. Polym. Sci. A, 1993, 31, 2432-2436) that 4-(2-propenyloxy)benzoic acid [4-(methoxycarbonyl)phenyl]ester or 4-(2-propenyloxy)benzoic acid [4-(butoxycarbonyl)phenyl]ester attached by a hydrosilylation reaction to a polysiloxane with a chain length of 35 units result in polymers with liquid crystalline phase, although the monomers are crystalline compounds.
No application for such polymers is proposed in the work of Chien et al. For the purpose of the present invention, the polymers published in this work are not applicable, because their viscosity is too high due to their high molecular weight, the glass transition temperature of the polysiloxane with side groups of 4-(2-propylenoxy)benzoic acid [4-(methoxycarbonyl)phenyl]ester is too high and the isotropic transition of the polysiloxane with side groups of 4-(2-propylenoxy)benzoic acid [4-(butoxycarbonyl)phenyl]ester is too low.